1. Field of the Invention
This invention relates to a hot-melt ink composition used in ink-jet recording apparatus. More particularly, it relates to a hot-melt ink composition capable of forming transparent images and to a hot-melt ink composition capable of forming opaque images.
2. Description of the Related Art
As ink-jet recording systems, various systems are proposed, e.g., what is called an electric field control system, in which electrostatic attraction is utilized to eject ink; what is called a drop on-demand system (or pressure pulse system), in which vibratory pressure of a piezoelectric device is utilized to eject ink; and what is called a thermal ink-jet system, in which a pressure produced by bubbles formed and grown by high heat is utilized to eject ink. These systems can provide very highly precise printed images.
In these ink-jet recording systems, water-based inks employing water as a main solvent and oil-based inks employing an organic solvent as a main solvent are commonly used. Printed images obtained using the water-based inks have a poor water resistance in general. On the other hand, the use of oil-based inks can provide printed images having a good water resistance.
However, these water-based inks and oil-based inks are liquid at room temperature, and hence have had disadvantages that feathering tends to occur when images are printed on recording paper and that no sufficient print density can be obtained. Also, because the inks are liquid, they tend to cause formation of deposits to cause a great lowering of the reliability of ink-jet recording systems because of, e.g., clogging of nozzles.
In order to overcome such disadvantages of the conventional inks of a solution type, what is called hot-melt oil-based ink-jet recording ink compositions, which are solid at room temperature and melt upon heating, are proposed.
Stated specifically, U.S. Pat. No. 3,653,932 discloses an ink containing a dialkyl sebacate. U.S. Pat. No. 4,390,369 and Japanese Patent Application Laid-open No. 58-108271 disclose an ink containing a natural wax. Japanese Patent Application Laid-open No. 59-22973 discloses an ink containing a stearic acid. Japanese Patent Application Laid-open No. 61-83268 discloses an ink containing an acid or alcohol having 20 to 24 carbon atoms and a ketone having a relatively higher melting point than these. Japanese Patent Application Laid-open No. 62-48774 discloses an ink containing a thermosetting resin having a high hydroxyl value, a solid organic solvent having a melting point lower than 150.degree. C. and a small quantity of a dye substance. Japanese Patent Application Laid-open No. 62-112627 discloses an ink comprised of a colorant, a first solvent which is solid at room temperature and is capable of liquefying upon heating to a temperature higher than the room temperature and a second solvent capable of dissolving the first solvent, being liquid at room temperature and being highly volatile. Also, Japanese Patent Application Laid-open No. 62-295973 discloses an ink containing a synthetic wax having a polar group and a dye soluble in the wax.
However, these hot-melt ink compositions have been developed without taking account of any relationship between the softening point of resin and the melting point of wax when the the resin and the wax are mixed.
Accordingly, under existing circumstances, any hot-melt ink composition suited for hot-melt ink-jet recording, having a good transparency on printing paper and OHP (overhead projection) sheets while satisfying both the basic performance required as ink and a good print quality, have not been produced while taking account of the relationship between the softening point of resin and the melting point of wax.
Meanwhile, when hot-melt ink compositions are made to adhere to OHP sheets and light is transmitted therethrough to project images onto screens, it is necessary to use an opaque hot-melt ink composition in order to surely form shadowed areas (black areas) on the projection screen. Similarly, when full-color projection images are formed, it is also necessary to use opaque hot-melt ink compositions so that areas printed in chromatic colors such as red, yellow and so forth on the OHP sheet can transmit light to form the corresponding chromatic areas on the projection screen, and at the same time the areas printed in black on the OHP sheet can surely form shadowed areas on the projection screen.
In conventional instances where such opaque hot-melt ink compositions are prepared, it has been attempted to incorporate a black dye in ink to the maximum of its solubility. However, the resulting ink composition can not be completely opaque, and it has been difficult to form black-and-white images on the projection screen.
Thus, under existing circumstances, any opaque hot-melt ink composition suited for hot-melt ink-jet recording, having a sufficient density on printing paper and OHP sheets while satisfying both the basic performance required as ink and a good print quality, have not been produced while taking account of the relationship between the softening point of resin and the melting point of wax.